1. Field of the Invention
The present invention relates to a method and apparatus for indexing ware to receive printing, and, more particularly, to a laser generator means to direct a laser beam on the surface of a rotating ware for generating an electrical signal upon detection of the beam by a detector at a predetermined indexing reference site to initiate a printing process in an area defined relative to the reference site.
2. Description of the Prior Art
Semi-automatic cylindrical printers are known in the art and take the form of a silk screen assembly that is reciprocated back-and-forth by a drive between a squeegee and a ware while the ware rotates in synchronism with the reciprocating screen through an interconnecting rack and pinion drive. If desired, the ware can be rotated solely by the contact with the moving silk screen. The squeegee is moved to establish line contact between the screen and the surface of the ware so that ink or other printing medium is forced by the squeegee through open spaces on the screen onto the ware. In this type of semi-automatic cylindrical printer the ware is loaded by a workman onto a holding fixture that can be vacuum operated or mechanically operated to establish a driving relation between the holder and the ware. The ware which can receive printing in this manner varies greatly and typically includes cylindrical items such as cosmetic bottles, pens, baseball bats, hats, aerosol cans and glass tumblers. Certain of these workpieces because of their configuration and material of construction, such as glass tumblers, which are cylindrical with a flat bottom, present problems which the present invention is designed to overcome when undertaking the printing on the cylindrical surface.
The blow molding process used to produce the glass tumbler presents ever changing variables from glass tumbler to glass tumbler including surface irregularities at the site where the tumbler is supported by the holder as well as along the entire cylindrical wall surface. In decorating machines using silk screen printing techniques for multiple colors, each color must be applied by a separate printing operation, thus necessitates a registration procedure between the existing printing and printing to be newly applied. Separate printing machines can be used for each one of the various colors or one or more printing machines can be used with a given set of silk screens being replaced at the completion of the printing on a select number of workpieces with all the screens being ultimately used to print all the required colors.
After the first color is printed, it is necessary to establish a desired position of the ware so that the site on the ware for colors to be subsequently printed will register properly with the existing printing. For this purpose it is necessary that the area on the ware to receive printing arrives at the site of the squeegee at the same time the open spaces in the screen are advanced across the squeegee to affect the printing operation. To accomplish such registration, it is known in the art to use a stylus, needle or similar pointed indicator mounted by a holder on the frame of the printer to identify the reference site on ware for establishing registration with indicia just prior to a the printing operation on the ware.
Because of the nature of the printing operation, the site where printing occurs being at the junction of the screen, squeegee and ware, it is not possible to use a pointing indicator at this site, thus the indicator must be used to locate a remote site as a point of reference. It is always necessary to establish registration in multiple color printing operations where an initial printing occurs in single stage printing machines is used to establish registration with a subsequent printing or printings.
The use of a pointed indicator presents certain problems to the establishment of registration of successive printings on ware which the present invention is designed to overcome. The holder drive for the ware must be jogged or hand manipulated into the position to establish registration with the pointer indicator. Such a registration procedure is time consuming and heretofore has a significant adverse affect to the printing cycle time. Moreover, the accuracy of registration is completely operator dependent and usually the accuracy achieved is within a tolerance of plus or minus three thirty seconds of an inch.
An attempt to improve the registration process was to replace the pointer indicator with an infrared light source and a detector. Erratic, random results were found when the color of the printing of the ware did not contrast sufficiently to produce a differential to the reflection which was necessary to trigger the response by the detector. In other instances, distortions to the infrared light due to dimensional changes to the material of the ware, which are always inherent with plastic, ceramic and vitreous materials. Even when the infrared light was detected, for the printing on one ware, there could be no assurances that the infrared detector could be relied upon to give faithful results for subsequent printing on the same or other ware. Irregularities to the surface configuration of the cylindrical part on the ware were also found to give irregular and inaccurate registration signals because the reflected infrared light energy is dispersed by the surface of the ware. Moreover, the use of infrared light source and detector necessitated a very close proximal relation between the source and the ware and the detector and the ware. As a result, the infrared source and the detector are prone to damage impact with the ware due to rotation of an out-of-round condition to the ware and during loading and unloading of the ware for the printing operation.
The loss of production is seriously affected when the misprinting of one color occurs. Usually all existing printings must be removed because it is impossible to remove only the misprinted pattern without the simultaneous removal of all colors. This condition is encountered with all printings irrespective of the ingredient used in the paint to solidify the paint after printing. This condition can be avoided by firing after each printing but substantial costs are added and can be avoided by the present invention.
Accordingly, it is an object of the present invention to provide an improved semi-automatic printing through the use of a laser beam generator and detector to accomplish registration of the ware relative to indicia on the silk screen for printing on the ware.
It is a further object of the present invention to use a laser beam and detector to avoid the short comings and disadvantages arising out of the use of known indicators for accomplishing registration of ware incident to a printing operation.